In the production of yarn from synthetic filaments, e.g. thermoplastic synthetic resin filaments, spinning machines are used which are capable of stretching and spinning the filaments or yarn. The filaments or yarn can be passed over or around so-called godets or galettes which can include heated drums with the filaments or yarn passing in one or more turns around the periphery thereof. The temperature of the surface of the godet, with which the synthetic threads are in contact, must be maintained within limited tolerances and held at certain values for suitable thread and yarn production and must, if the properties of the thread or yarn to be maintained constant and constant processing speed is to be maintained, also be held constant.
As a consequence, known spinning machines for this purpose usually include one or more sensors capable of detecting temperatures of the surface of the godet contacted by the yarn or thread and which outputs a signal utilized for control circuitry. The control circuitry, in turn, can operate a heating element of the godet or a heating element capable of transferring heat to the godet and thus a heating element or unit which can be located within the godet or outside the godet and which serves to maintain a desired set point temperature within certain limits.
The temperature sensor can be arranged in various ways. For example, the temperature sensor can be embedded in the surface of the godet (DE OS 21 08 825). In this case, however, only one location along the periphery of the godet can be sensed so that different temperatures over the godet surface usually cannot be detected. It has been proposed, therefore, to provide annular temperatures sensors which can be integrated in a groove of an inner surface of a godet (DE-OS 1 901 902). These overcome the aforedescribed drawback, but the sensor signal must be conducted from the rotating godet to the stationary part of the machine and can involve expensive and unreliable slip rings and a signal transmission path which is prone to mechanical failure or in the case of wireless signal transmission, may suffer from the development of noise or other perturbations.
DE 40 24 432 C2 describes a process for determining the temperature of an inductively heated machine part in which the heated element is included in or has a secondary heating winding which is inductively coupled to a primary winding. An electronic current is passed through the primary winding and induces the heating current in the secondary winding. The temperature of the heated element can then be determined by a measurement of the current passing through the primary winding and by a measurement of the voltage generated across the secondary winding. In a calibration process, the ohmic resistance R.sub.2 of the heated machine part or heating element is determined as a function of its temperature T and is calculated from the relationship T=f.sub.T (R.sub.2) as stored. The current I.sub.L through the main inductance is measured as a function of the voltage drop U.sub.L and the function I.sub.L =f.sub.L (U.sub.L) is calculated and stored. As a measurement of the voltage drop at the main inductance, the induced voltage in the secondary measurement winding is used. In operation, the temperature T can then be calculated in accordance with the relationship T=f.sub.T {U.sub.L /(I.sub.A -f.sub.L (U.sub.L))} (see DE 40 24 432 C2).
This process has, however, the drawback that a measuring secondary winding is required and, in addition, the calibration step requires an extremely large number of points in order to enable the temperature dependency of the ohmic resistance of the heated element to be determined. The same applies for the measurement of the current through the main inductance as a function of the voltage drop at the main inductance.